Generally, in the manufacture of semiconductor integrated circuits, each of film forming process, etching process, oxidation/diffusion process is provided to objects to be processed such as semiconductor wafers, etc. With the recent progress of high integration, high refining and thinning, improvement in the film quality in the above processes is the most important subject. Under such circumstances, as a film forming method for obtaining a good quality film an atomic layer deposition (hereinafter referred to as ALD) method is developed.
In the ALD, by use of a difference between absorption energy of a first layer of raw gas applied to an absorption surface and absorption energy of a second layer and the following, desired films are deposited one layer by one at the atomic level or molecular level. More specifically, temperature and pressure are controlled at the film forming time. In other words, the rise and fall in temperature and pressure are repeated and the film is formed while removing the excessive raw gases of the second layer and the following layers.
The following explains the ALD as an example in the case where a titanium nitride (TiN) film is formed using titanium tetrachloride (TiCl4) and ammonia (NH3) as raw gases.
The thermal treatment apparatus that performs the ALD is disclosed in Unexamined Japanese Patent Publication Nos. 6-244143, 7-78766 and 7-153706. FIG. 6 shows one example of the structure of the thermal treatment apparatus for performing the ALD.
As illustrated in FIG. 6, a thermal treatment apparatus 102 includes, for example, an aluminum chamber 104 with a substantially circular cross section. The diameter of a lower portion of the chamber 104 is formed to be smaller than that of an upper portion. The capacity of the interior of the chamber 104 is provided as small as possible, so that high exhaust efficiency can be obtained. At a side wall of the chamber 104, there is formed a quartz nozzle 106 for introducing a raw gas. The raw gas is supplied to a treatment space S through the nozzle 106.
At the side wall of the chamber 104, there is formed a gate valve 108 for loading/unloading a semiconductor wafer as an object to be processed on/from the chamber 104. The gate valve 108 is airtightly openable and closable.
At the lower portion of the chamber 104, there is formed a lower-portion space 110, which is narrower than the upper portion, as mentioned above. A hollow cylindrical shaft 112 stands from the bottom of the chamber 104 to pass through the lower portion space 110, and a joint between the chamber 104 and shaft 112 is seated by a seal material 114 such as an O ring and the like.
A disc-like mounting table 116 having a thickness t1 of a few cm is fixed to an upper end portion of the shaft 112. On the upper surface of the mounting table 116, a semiconductor wafer W is mountable. Moreover, the mounting table 116 includes a heater 118, which is formed of a resistance member placed in a predetermined pattern in its interior. The mounting table 116 is formed of sintered ceramics of, for example, aluminum nitride. The shaft 112 is formed of he same material as that of the mounting table 116, that is, aluminum nitride, and is joined to the mounting table 116 by solid-state welding 120. Moreover, in the mounting table 116, a lift pin 126 is provided to pass therethrough and to be movable up and down by an air cylinder 128.
The heater 118 is connected to a feeder line 122 passing through the hollow shaft 112, so that power is supplied to the heater 118 via the feeder line 122. Here, an interior of the shaft 112 is set to be an atmospheric state, heat of the feeder line 122 is sufficiently radiated to make it possible to prevent occurrence of burning.
A length L of the shaft 112 is set with consideration given to heat resistance of the seal material 114 formed at the lower end. More specifically, the length L1 is set to, for example, about 30 cm to ensure a sufficient temperature difference between the upper and lower ends where the mounting table 116 is provided. Moreover, at the bottom of the chamber, there is formed a cooling jacket 124 into which cooling water flows in order to protect the seal material 114.
At a lower portion side wall of the chamber 104, there is formed an exhaust port 130 communicating with the lower portion space 110. The exhaust port 130 is connected to an exhaust pipe 132 connected to an exhaust device (not shown). The exhaust device makes it possible to set atmosphere in the chamber 104 including the lower portion space 110 to be a high vacuum state.
An explanation is next given of the process for forming a TiN film by the ALD using the aforementioned thermal treatment apparatus 102.
First, the mounting table 116 is maintained at temperature that is suitable for adhering TiCl4, for example, 600° C., and TiCl4 gas is introduced into the chamber 104 for a short time period, for example, a few seconds. Here, the TiCl4 gas may be introduced thereinto together with carrier gas as necessary. As a result, a TiCl4 molecular layer is adhered onto the surface of the wafer W in a multilayer form.
Next, the interior of the chamber 104 is exhausted up to a high vacuum of, for example, about 1.33×10−3 Pa (10−5 Torr), and the temperature of the mounting table 116 is reduced to temperature that is suitable for adhering NH3, for example, 300° C. in this exhausting process, the TiCl4 molecular layers adhered onto the surface of the wafer W are scattered due to the absorption energy difference as leaving the first molecular layer. As a result, the TiCl4 molecular layer of one layer is adhered onto the surface of the wafer W.
In a state that pressure in the chamber 104 reaches about 1.33×10−3 Pa and the temperature of the mounting table 116 is reduced to about 300° C. by such exhausting, the NH3 gas is introduced into the chamber 104 for a short time period, for example, a few seconds. By the introduction of the gas, pressure in the chamber 104 is returned to about 133 Pa (1 Torr). Here, the NH3 gas may be introduced thereinto together with carrier gas as required. Accordingly, the TiCl4 molecular layer of one layer on the wafer surface and the NH3 gas are reacted with each other to form a TiN layer of one layer, and an NH3 molecular layer is adhered onto an upper surface of the TiN layer in a multilayer form.
After that, the interior of the chamber 104 is exhausted up to about 1.33×10−3 Pa and the temperature of the mounting table 116 is increased to, for example, 600° C. At this time, the second NH3 molecular layer and the following are scattered excepting the first NH3 molecular layer adhered onto the surface of the TiN film.
Next, a TiCl4 gas is introduced into the chamber 104 for a few seconds. At this time, the NH3 molecular layer of one layer on the TiN film and the TiCl4 gas are reacted with each other to form a one-layered TiN films and a TiCl4 molecular layer is adhered onto this TiN film in a multilayer form. Accordingly, a two-layered TiN film is formed on the surface of the wafer W.
Afterwards, the same operations as mentioned above, that is, the supply and exhaust of each raw gas and the temperature rise and fall of the mounting table 116 are repeated the predetermined number of times, and the TiN films are deposited one layer by one to as to obtain a TiN film with a desirable thickness. The aforementioned operations are repeated, for example, 100 to a few hundreds times.
As mentioned above, according to the ALD, since the films can be formed one lay by one, the film thickness can be controlled with high accuracy. Moreover, the film with high quality can be obtained as a whole. Furthermore, since the films can be deposited one molecular layer by one, it is possible to provide a gradient in the characteristic; for example, the film quality is gradually changed.
By the way, as explained above, in the ALD, it is required that the temperature rise and fall of the mounting table 116 and the supply and exhaust of the gas to/from the chamber 104 should be repeated many times. For this reason, in order to obtain high productivity and throughput, the temperature rise and fall and the exhaust must be performed for a short time period and at high speed.
However, in the aforementioned treatment apparatus 102, since the thickness t1 of the mounting table 116 is a few cm, the heat capacity is relatively large and much time is required for the temperature rise and fall. The temperature fall of the mounting table 116 is performed by escaping heat to the cooling jacket 124 provided at the bottom of the chamber 104 through the shaft 112 jointed thereto. However, thermal conductivity of the shaft 112 formed of ceramics is relatively low. Even in this point, much time is required the temperature rise and fall of the mounting table 116.
In order to improve the temperature fall rate, a reduction in the length of the shaft 112 and the use of material having a good thermal conductivity can be considered. However, if the length of the shaft 112 is too short, the seal material 114 with heat-resistant temperature of about 150° C. to 200° C. is damaged by heat. Moreover, if other material having a different linear expansion coefficient from the ceramic mounting table 116 is used in the shaft 112, breakage occurs in the vicinity of the joint 120 therebetween.
Furthermore, if the length L1 of the shaft 112 is ensured to some degree, the volumetric capacity of the lower portion space 110 is increased, so that the entire volumetric capacity of die chamber 104 is also increased. As a result, much time is required to attain exhaust to a predetermined vacuum state.
Thus, in the conventional thermal treatment apparatus, there was a problem that much time was required for the temperature rise and fall of the mounting table and the object to be processed and much time was also required for vacuum exhaust of the interior of the chamber.